Switch macOS CI to use Python from Azure images

Fixes Python 3.10 and should be faster not having to download & install
Python and pip.

This also removes Python 3.6 from macOS CI - the Azure hosted version of
Python 3.6 has a problem with linker flags (it is setting -stack_size) and
can't compile Python extensions.  Python 3.6 is going to be EOL in a month or
so, so it seems fine to just remove it from CI.

.azure/ci-macos-job.yml

@@ -6,26 +6,14 @@ jobs:
     vmImage: 'macOS-1015'
   strategy:
     matrix:
-      Py36:
-        python_version: '3.6.8'
-        python_pkg: 'python-3.6.8-macosx10.9.pkg'
-        python: 'python3.6'
       Py37:
-        python_version: '3.7.9'
+        python.version: '3.7'
-        python_pkg: 'python-3.7.9-macosx10.9.pkg'
-        python: 'python3.7'
       Py38:
-        python_version: '3.8.10'
+        python.version: '3.8'
-        python_pkg: 'python-3.8.10-macosx10.9.pkg'
-        python: 'python3.8'
       Py39:
-        python_version: '3.9.9'
+        python.version: '3.9'
-        python_pkg: 'python-3.9.9-macosx10.9.pkg'
-        python: 'python3.9'
       Py310:
-        python_version: '3.10.0'
+        python.version: '3.10'
-        python_pkg: 'python-3.10.0post2-macos11.pkg'
-        python: 'python3.10'
 
 
   variables: {'PYTHONUNBUFFERED': '1'}
@@ -34,36 +22,30 @@ jobs:
   - checkout: self
     submodules: recursive
 
-  - script: |
+  - task: UsePythonVersion@0
-      curl -O https://www.python.org/ftp/python/$PYTHON_VERSION/$PYTHON_PKG
+    inputs:
-      sudo installer -pkg $PYTHON_PKG -target /
+      versionSpec: '$(python.version)'
-      which $PYTHON
-      $PYTHON --version
-    displayName: 'Fetch Python'
 
   - script: |
-      curl -O https://bootstrap.pypa.io/get-pip.py
+      python -m pip install -U setuptools wheel
-      $PYTHON get-pip.py
+      python -m pip install -r requirements.txt
-      $PYTHON -m pip install -U setuptools wheel
-      $PYTHON -m pip install -r requirements.txt
     displayName: 'Install Python dependencies'
 
   - script: |
-      #sudo xcode-select -s /Applications/Xcode_9.4.1.app/Contents/Developer
       xcode-select --print-path
-      $PYTHON build.py --jobs=4 build_wx
+      python build.py --jobs=4 build_wx
     displayName: 'build wxWidgets'
 
   - script: |
-      $PYTHON build.py dox etg --nodoc sip
+      python build.py dox etg --nodoc sip
     displayName: 'generate code'
 
   - script: |
-      $PYTHON build.py --jobs=4 build_py
+      python build.py --jobs=4 build_py
     displayName: 'build wxPython'
 
   - script: |
-      $PYTHON build.py bdist_wheel
+      python build.py bdist_wheel
-      $PYTHON build.py install
+      python build.py install
-      $PYTHON -c "import wx; print(wx.version());"
+      python -c "import wx; print(wx.version());"
     displayName: 'build wheel and test install'